1038 
ANIMAL RESOURCES 
Pneumobag'*' connected to a partially open re- 
breathing circle with a soda lime cannister. The 
Pneumobag permits positive pressure ventila- 
tion adjusted to an inflation pressure of 25 to 30 
cm H2O at a frequency of 15 to 20 per minute. 
The anesthesia machine is adjusted to deliver 
0.25 to 0.75% halothane and 50% nitrous oxide 
using oxygen as diluent. Atropine is given in- 
travenously in a dose of 0.01 mg/kg and meper- 
idine hydrochloride is administered intramuscu- 
larly in a dose of 1 mg/kg. An indwelling 
catheter in the jugular vein is connected to an 
infusion bottle containing 200 mg succinylcho- 
line and 50 mg meperidine hydrochloride in 250 
ml of 5% dextrose in water. Generally, 250 to 500 
ml are used to maintain immobility and analge- 
sia during extracorporeal circulation and car- 
diac surgery. 
The lungs are statically inflated to a pressure 
of 25 to 30 cm HoO during the period of bypass 
and are reinflated every 20 to 30 minutes dur- 
ing bypass. When extracorporeal circulation is 
concluded, the animal is returned to the inhala- 
tional anesthetic regimen detailed above. The 
nitrous oxide-oxygen-halothane mixture is con- 
tinued until the pleura are closed. During clo- 
sure of the chest wall the animal is allowed to 
breathe only 50% nitrous oxide-50% oxygen. 
Within 15 to 20 minutes of completion of sur- 
gery, ventilation is carried out with 100% oxy- 
gen with an Ohio Medical volume cycled 
ventilator'^ with the following control settings : 
tidal volume — 10 to 15 ml/kg body weight ; rate 
— 15 to 20 cycles per minute; deep breath — 50 
to 100% of tidal volume added to the tidal vol- 
ume every two minutes. This ventilator also ul- 
trasonically humidifies its output, thus prevent- 
ing drying of the airway. 
Only two animals have died as a direct conse- 
quence of anesthesia in over 100 anesthetic pro- 
cedures carried out with variants of the above 
described technique. Both of the animals devel- 
oped irreversible ventricular fibrillation. Induc- 
tion time is usually five minutes or less. When 
anesthesia becomes stabilized, blood pressure is 
always above 110/70 with occasional values ex- 
hibiting elevation to hypertensive levels of 
180/130 or greater. Arterial blood gases have 
" Ohio 560 respirator, Ohio Medical Products, Madison, Wisconsin 
53701 
ranged as follows: Pa02, >100 mm Hg; 
PaCOo, 35 to 40 mm Hg; and pH 7.35 to 7.45 
Bradycardia with heart rate falling to 50 beats 
per minute was observed in a few calves. Hypo- 
tension was not a problem so long as halothane 
concentration was kept below 1.0%. 
PULMONARY VENTILATION 
In our early studies a Bird Mark 8 or Mark 
10 pressure cycled respirator was used to venti- 
late the lung of calves which weighed from 70 
to 84 kg. Positive pressure ventilation was car- 
ried out at a cycling rate of 15 per minute with 
the pressure adjusted to 20 cm H2O. At this 
juncture, we took note of the work of McLaugh- 
lin and associates,^ who compared pulmonary 
anatomy in calves, cows, sheep, pigs and other 
animals and indicated that calves had well-de- 
veloped secondary lobules with marked interlob- 
ular supporting tissue and thick pleura. During 
thoracotomy, the thick pleura and extensive 
supportive tissue created a resistance to expan- 
sion greater than that in lungs without these 
characteristics, such as in lungs of man and 
dogs. Thus, we assumed that higher airway 
pressures would be required to obtain a similar 
degree of pulmonary gas exchange in calves 
than in dogs or man. In our subsequent experi- 
ments in larger calves, an Ohio Medical 560 vol- 
ume-cycled respirator was used. Tidal volume 
was adjusted so that inflation pressure approxi- 
mated 25 to 30 cm H2O to achieve more com- 
plete lung expansion. The larger tidal volume 
coupled with the intermittent deep breaths de- 
livered by this ventilator apparently reversed 
developing atelectasis. Observations at necropsy 
confirmed the fact that there was less hemor- 
rhagic atelectasis in animals thus ventilated 
during the postoperative period than in those 
animals ventilated with the pressure-cycled re- 
spirator. Our observations at necropsy con- 
firmed those of Donawick and coworkerSj^" 
who found that calves ventilated with minute 
volumes between 30 and 40 liters per minute did 
not develop pulmonary atelectasis which was 
observed by them in calves ventilated with 
smaller minute volumes of 15 to 20 liters per 
minute. 
